An O3 production apparatus that produces O3 to be supplied to the exhaust gas of an internal combustion engine by generating a plasma has been known. Supply of O3 produced by such an O3 production apparatus to the exhaust gas promotes oxidation of NO in the exhaust gas, whereby NO can be changed into NO2. NO2 is stored more easily in an NOx storage reduction catalyst (which will be simply referred to as an “NOx catalyst” hereinafter) as compared to NO. Therefore, in the case where an NOx catalyst is provided in the exhaust passage of the internal combustion engine, changing NO in the exhaust gas into NO2 can promote storage of NOx in the NOx catalyst.
Furthermore, in the case where a particulate filter (which will be simply referred to as a “filter” hereinafter) that traps particulate matter (which will be hereinafter referred to as “PM”) in the exhaust gas is provided in the exhaust passage of the internal combustion engine, supplying O3 produced by the O3 production apparatus to the filter when removing PM trapped in the filter can promote oxidation of PM trapped in the filter.
Japanese Patent Application Laid-Open No. 2006-257948 discloses a technology in which PM in the exhaust gas is absorbed by a honeycomb structure by the effect of an electrical field. This Japanese Patent Application Laid-Open No. 2006-257948 also discloses a technology in which O3 is produced by an O3 production apparatus including a discharger and a pulse battery, and O3 thus produced is supplied as an oxidation prompter to the honeycomb structure.
Japanese Patent Application Laid-Open No. 2004-353619 discloses a technology in which NO in the exhaust gas is changed into NO2 by O3 produced by generating a plasma, and NO2 is reduced into N2 by an NO2 catalyst. Japanese Patent Application Laid-Open No. 2001-73748 discloses a technology in which the quantity of NO2 is increased when the temperature of an NOx catalyst is low.